1. Field of the Invention
This invention relates to an interior rotor for an electric machine. In particular, this invention relates to a bipolar permanent magnetic field exited interior rotor for an electric machine. More particularly, this invention relates to an interior rotor for an electric machine having sheet metal segments disposed on a shaft, the shaft being shaped many-sided (as opposed to round) at least in that area where the segments are disposed thereon.
2. Description of the Prior Art
German utility model No. 77 26 439 discloses a bipolar permanent-field interior rotor for an electric machine in which a shaft is used the middle section of which is shaped many-sided. The magnetic poles of the rotor are formed by sheet metal segments which are arranged on the many-sided section of the shaft. Between the segments and the shaft are inserted permanent magnet slabs which abut the sides of the shaft. The axes of magnetization of the magnet slabs are arranged radially with regard to the axis of rotation.
In the previously known interior rotor a shaft having six sides is used. Each side of the shaft is covered by one of the permanent magnet slabs. In other words, the number of permanent magnet slabs equals the number of sides of the shaft.
In case that there shall be used rare earth magnets (containing elements of the lathanide series) for the permanent magnet slabs in such a rotor, the following problem will arise.
Because of the good magnetic properties of the rare earth magnets only a relatively small magnet volume is required for the generation of a sufficiently strong magnetic field. The magnetic slabs, therefore, can be designed to be thin, for instance, 2 mm. Due to the brittleness of the rare earth magnets, however, and in order to obtain sufficient mechanical strength or stability, the thickness should not be less than a certain minimum value, for instance, 3 to 4 mm. By virtue of this greater thickness of the magnets, an improvement of the magnetic conditions of the electric machine cannot be obtained. On the contrary, expensive magnetic material will be used without any effect.